1. Field of the Invention
The present invention relates to an optical scanner for retrieving digital images of opaque and transmissive objects. In particular, this invention relates to a scanner in which the alignment of a transmissive mode light source and an image-retrieving device can be synchronously maintained.
2. Description of the Related Art
Recent developments in the use of high speed, low cost digital electronic imaging systems to process, transmit and display digital information have substantially increased the demand for optical scanners to convert information into digital electronic form. A variety of optical scanners, including facsimile machines and digital copiers, have been developed to meet this growing demand. The optical scanners generally fall within two categories: reflective scanners for scanning opaque objects, such as documents and photographs; and transmissive scanners for scanning transparent objects, such as slides and transparencies. Reflective scanners project light onto the side of the object to be scanned. The object illuminated by the reflected light is then viewed by a digital image-retrieving device that converts the image into a digital electronic form. Transmissive scanners project light through the object to be scanned. The transmissive object is also viewed by a digital image-retrieving device to then be stored in a digital electronic form. The growing demand for both reflective and transmissive scanners has resulted in a need for scanners to provide both capabilities.
AA conventional dual mode scanner is illustrated in FIG. 2. This type of scanner includes a digital image-retrieving device 20 and a reflective mode light source 18 mounted on rails and driven by a motor 14 and belt 16 within the scanner body to scan along the length of the document. The reflective mode light source 18 projects light upon the portion of the object within the field of view of the image-retrieving device. Documents and photographs are placed on a transparent plate 22. The reflective mode light source 18 and image-retrieving device 20 move incrementally lengthwise across a document placed on the transparent plate 22 to scan an image line by line.
This conventional dual mode scanner has a second light source 28 for scanning transmissive objects. This transmissive mode light source 28 is preferably housed behind a second transparent plate (not shown) within a scanner lid that overlies the scanner body. The transmissive mode light source 28 is movably mounted on a pair of rails and wired electronically to the scanner body to provide light through a portion of a transmissive object within the field of view of the image-retrieving device. The transmissive mode light source mechanism includes a second motor 24 and belt 26 to maintain the light directly within the image-retrieving device's field of view by incrementally moving along the rails in the scanner lid in synchronous movement with the image-retrieving device 20.
In order to obtain the best picture quality, the transmissive mode light source 28 located in the scanner lid must be maintained in synchronous alignment with the image-retrieving device 20 located in the scanner body at all times during scanning. Referring to FIGS. 1a and 1b, if transmissive mode light source 28 does not move synchronously with the image-retrieving device 20, the relative positions of the transmissive mode light source 28 and the image-retrieving device 20 will not be maintained. Thus, the light intensity detected by image-retrieving device 20 will change, leading to a deterioration of the scanning quality.
The conventional dual mode (transmissive mode/reflective mode) scanner relies on the operation of two motors 14, 24 and two belts 16, 26. Therefore, in addition to the extra cost of the second motor and belt, any aberration in the functioning of either motor or a slippage of either belt could lead to non-synchronous movement of the transmissive mode light source 28 and image-retrieving device 20. Furthermore, such a scanner typically requires electronic circuitry in both the scanner body and the scanner lid to maintain the alignment between the transmissive mode light source 28 and the image-retrieving device 20. This further increases the cost of manufacturing the scanner.